Paul_KD7HB:
Is there some problem with making probes the old fashioned way? Platinum wire sealed in a thin glass tube.
Paul
Price is one. Those are expensive, and just about impossible to build yourself.
The sensor I'm developing I expect to sell at well under USD 10, probably less than half that. It'll be open source & open hardware so the DIY crowd can try and build their own if they want.
Eagerly awaiting PCB prototypes with just this sensor. Gotta get them soon, will definitely have to test them in sea water kind of concentrations (55 mS/cm is 27500 ppm of NaCl in water; trivial to make).
I'm now using a 10 nF capacitor, and have a time of just over 2 microseconds for the 1840 ppm solution. At a 12.5 ns resolution that's comfortable. It also makes for an overall AC frequency of about 250 kHz.
To get to 27500 ppm, one would need a capacitor of 150 nF to get to the 2 us discharge times. Of course at the other end of the scale, it's going to be slower to discharge with lower frequency.
The problem of the AC frequency: under about 3 kHz and above about 300 kHz it's not linear. It's just a little off down to 1 kHz, and you can go to maybe 500 kHz or even 1 MHz on the upper end, but that's it. The accurate range is about two order of magnitude, an extended range would be nearing three orders of magnitude.
This means a 0.5-55 mS/cm range is no problem, but I will have to use a bigger capacitor and not sure if that's going to fit that well in my tiny tiny probe (for more stable timing in this very wet environment, I've opted for film capacitors rather than smaller, cheaper ceramics). It could even give your an extended range of 0.1-100 mS/cm - total three orders of magnitude.
For my hydroponic application I'm going to use a 22 nF capacitor, to push the upper range a bit, as I hope to end up at a 0.1-10 mS/cm range. Typical hydroponic solutions are 1-2 mS/cm.